Generally, a communications system has a management and maintenance channel. Management and maintenance channels of some systems are embedded in data channels, and therefore, reliability of the management and maintenance channels is the same as reliability of the data channels. To improve reliability of management and maintenance channels of some systems, some methods for improving the reliability are specially used. For example, SNR (signal-to-noise ratio) margins of some subcarriers are increased, and these subcarriers are used to form an independent channel. The channel is referred to as an RC (robust channel) and these subcarriers are referred to as RC subcarriers. In some communications systems, channels with different reliability may be set up according to importance of service data that needs to be transmitted, so as to carry services with different reliability requirements. For example, a VoIP (Voice over Internet Protocol) service has a small data volume and requires high reliability and a low delay, and a low-speed RC channel with a low delay and high reliability may be set up in a system for the VoIP service. However, how to transmit data by using the RC channel is a problem that needs to be resolved.
In a solution in the prior art, an architecture of a signal modulation system includes a PMS-TC (physical media specific transmission convergence) layer and a PMD physical media dependent) layer. In a transmission period of each DMT (discrete multi-tone) symbol, the PMS-TC layer writes multiple pieces of bit data to the PMD layer. When the signal modulation system supports an RC channel, an interface between the PMS-TC layer and the PMD layer may be divided into an RC channel and a service data channel, and RC bit data and service bit data are respectively transmitted through the two channels in the transmission period of each DMT symbol. After a DMT data frame is formed by using the RC bit data that is obtained from the RC channel and the service bit data that is obtained from the data channel, if all bit data in the DMT data frame is coded, a DMT data frame coding rate is affected, and a throughput during decoding is high; and if bit data in the data frame is not coded, but is directly mapped onto a service subcarrier, information transmission security cannot be ensured because no coding is performed to protect the data frame.